Page 114 - CIWA Water Data Revolution Overview Report

= 0.0019 ∙ ∙ ( + 21.0584) ∙ ( − 0.0874 ∙ ) 0.6278
0

Where:
• Tavg: Daily average temperature (°C)

• T Dj: Daily difference between maximum and minimum temperature (°C)
• Pj: Daily precipitation (mm)

• S0: Extraterrestrial solar radiation (mm/day)

The daily PET estimates were aggregated to annual values for further analysis. Current climate

data used in baseline modelling (ET and PET from the SSEBop v52, GLEAM v43 and

precipitation from CHIRPS4 database) were used to derive a relationship between
precipitation, ET, and PET under current conditions following the Budyko framework as

detailed by Gunkel and Lange (2017):

1

= 1 + − [1 + ( ) ]

Where is a dimensionless parameter that reflects basin-specific characteristics. For the
Volta River Basin, was calibrated to 1.8, consistent with the literature for semi-arid
regions.

3.8 Analysis of Projected Climate Scenarios

Figure 5 depicts the monthly mean precipitation trends for the baseline period (2003–2021)
and future projections (2025–2050) based on the GFDL-ESM4 and MPI-ESM1-2-HR models.

The uni-modal distribution is evident across all scenarios, with a peak in August, reflecting the

region's characteristic rainy season. A dip is observed during November–December and

January–February, corresponding to the drier periods of the year.
The GFDL-ESM4 model projects a slightly reduced precipitation peak in August compared to

the baseline, suggesting a decrease in overall wet season rainfall. Conversely, the MPI-ESM1-

2-HR model projects an increased peak rainfall in August, indicating a wetter wet season

relative to the baseline. Despite these variations in magnitude, both models maintain similar
seasonal patterns, reinforcing the persistence of uni-modal precipitation dynamics in the

Volta River Basin.

2 Actual ET (ETa) is produced using the operational Simplified Surface Energy Balance (SSEBop) model (USGS - FEWS NET Daily Actual
Evapotranspiration (ET))
3 https://www.gleam.eu/
4 The Climate Hazards Group, The University of California, Santa Barbara, (https://www.chc.ucsb.edu/data/chirps)